It is known to bleach hair with the aid of a paste obtained by mixing, at the time of application to the hair, a bleaching composition based on peroxidized derivatives with water or, better still, with hydrogen peroxide. The bleaching composition consists, in a known manner, of a peroxidized derivative, generally a persulphate or a perborate of sodium, potassium or ammonium and sometimes a percarboxylic acid salt or a peroxide of, for example, barium or strontium. These compositions also contain, in a known manner, strongly alkaline agents such as alkali metal or alkaline-earth metal metasilicates, phosphates or carbonates; they may also contain other additives: agents for controlling the release of oxygen during the mixing with hydrogen peroxide, such as magnesium carbonate or magnesia; surface-active agents, such as fatty alcohol sulphates, alkyl sulphates and alkylbenzenesulphonates; thickening agents, such as cellulose derivatives, for example carboxymethyl cellulose, starch and its derivatives, guar gum, xanthan gum and alginates; blue or violet colouring agents and perfumes. Such bleaching compositions are described, for example, in "The science of hair care" by C. Zviak, Marcel Dekker Inc. 1986, pages 225 and 226.
The bleaching composition is often used in the form of a powder of small particle size, which makes possible an easy and rapid dissolution in hydrogen peroxide. However, these pulverulent compositions have several disadvantages. In the first place, the pulverulent compositions consist of powders having different apparent densities and, in the course of their handling and their storage, separation of the constituents occurs, the heavier collecting at the lower part of the packaging in which the composition is contained and the lighter in the upper part; as a consequence, during withdrawal of the composition in order to mix it with hydrogen peroxide, the volumes taken at the upper part of the packaging and those taken at the lower part have different compositions and, therefore, a different bleaching power. In the second place, the compositions in pulverulent form give off, during their handling, dusts which contain peroxidized derivatives and are, as a consequence, strongly irritating to the lungs.
In order to resolve this problem, it has already been proposed to place the bleaching composition in granulated form.
According to WO-A 92/03120, it has been proposed to granulate a persulphate and to mix it with optionally granulated particles of the various other constituents of the composition. Granulation of the persulphate may be performed either by spraying and drying an aqueous persulphate solution optionally containing surface-active agents or water-soluble thickening agents, or by spraying a solution of surface-active agent or of thickening agent onto a moving bed of solid persulphate. If this process, as presumed, makes it possible to avoid the formation of irritating persulphate dusts, it certainly does not make it possible to resolve the problem of the separation of the particles of the various constituents of the bleaching composition.
In FR-A 2,044,324, it has been proposed to granulate the constituents of the pulverulent bleaching composition collectively with the aid of a binder: polyvinylpyrrolidone or glucose dissolved in an aqueous, aqueous-alcoholic or alcoholic medium. The problem of the separation of the various constituents of the bleaching composition is thereby resolved, since the various constituents are present in the same granule. However, it has been observed that the use of polyvinylpyrrolidone alone as binder gives hard, but nevertheless crumbly, granules which, on repeated rubbing, form fine dusts which are capable of being carried into the atmosphere. Furthermore, granules of high particle size between 1 and 6 mm are obtained, which increases the dissolution time and gives, on mixing with hydrogen peroxide, a paste which remains granular for a long time and the application of which is unpleasant; furthermore, on application to hair this paste runs the risk of giving inhomogeneous and non-reproducible bleaching.